pH Lockout

pH lockout is the most common form of nutrient lockout in cannabis cultivation and the most preventable. The mechanism is simple: each macro and micro nutrient is only available to plant roots within a specific pH range, and pH drift outside that window makes the nutrient chemically inert in the soil solution regardless of how much is present. The cannabis nutrient availability chart (Mulder's Chart adapted for cannabis): Nitrogen — bioavailable pH 6.0-8.0; Phosphorus — pH 6.5-7.5 (locks out at extreme low and high pH); Potassium — pH 6.0-7.5; Calcium — pH 6.5-8.5 (locks out below 6.0); Magnesium — pH 6.5-8.5; Sulfur — pH 6.0-9.0; Iron — pH 4.5-6.5 (locks out above 7.0); Manganese — pH 4.5-6.5; Zinc — pH 5.0-7.0; Copper — pH 5.0-7.0; Boron — pH 5.0-7.0; Molybdenum — pH 7.0-8.5. Cannabis grown in hydroponics targets pH 5.5-6.0 to keep all major nutrients in their bioavailable windows simultaneously. Soil targets pH 6.0-7.0 because soil microbiology buffers pH naturally and provides a wider tolerance. Causes of pH drift: incorrect water source pH (well water can run 7.5-8.5, RO water can crash to 5.0), nutrient solution chemistry (heavy phosphorus additions push pH down, heavy potassium up), microbial action in soil/coco changing local pH over time, salt buildup interacting with new feeds. Diagnosis and treatment: test runoff pH after watering — if runoff is outside 5.8-6.5 (hydro/coco) or 6.0-6.8 (soil), flush with pH-corrected water at 2x container volume and adjust future feeds to compensate.